This invention relates to a transmitter for use in a communication system which carries out communication by the use of a pilot signal and, in particular, to a communication apparatus used in a transmitter of a mobile communication system.
In a mobile communication system of a type described, communication is carried out through a forward link or a downward link between a base station and a mobile station or stations located within a service area of the base station. Specifically, the base station transmits data or information signals to the mobile station through the forward link while each mobile station transmits data or information signals to the base station through the backward link. The forward and the backward links may be referred to as down and up links, respectively.
The following description will be mainly directed to the communication which is carried out from the base station to the mobile station through the forward link because the present invention is not directly concerned with communication of the backward link.
Recent interest has been directed to a communication system which uses a code division multiple access (CDMA) technique and which will be called a CDMA system hereinafter. In such a CDMA system, a common frequency is assigned to a plurality of base stations each of which has a peculiar psuedo random number or noise (PN) code. In other words, each base station is specified by a peculiar PN code allocated thereto.
Furthermore, a pilot channel, a sync channel, a paging channel, and a traffic channel are prepared as code channels in the forward link of the CDMA system. In this event, the pilot channel serves to acquire and maintain synchronization in each mobile station and to reproduce a sequence of clocks in each mobile station while the sync channel serves to adjust a PN code between the base and the mobile stations. In addition, the paging channel is used to transmit paging information and hand-off information to each mobile station while the traffic channel is used to transmit a speech signal to each mobile station.
Herein, it is known in the art that the PN code is multiplied by Walsh codes which are different from one another to form spread codes which specify the pilot channel, the sync channel, the paging channel, and the traffic channel. For example, the pilot channel is defined by multiplying the PN code by the Walsh code 0 which is specified by a sequence of zeros. As a result, the PN code itself is transmitted through the pilot channel. In addition, the sync and the paging channels are specified by Walsh codes 32 and 1, respectively, while the traffic channel is specified by Walsh codes n where n is an integer except 0, 1, 2, 3, 4, 5, 6, 7 and 32.
From the above, it appears that a pilot signal arranged in the pilot channel should be correctly reproduced so as to carry out correct coherent detection of any other signals than the pilot signal in each mobile station. Otherwise, the signals arranged in the sync, the paging, and the traffic channels are not accurately reproduced because a phase reference of the coherent detection is not given to the reproduction of the other signals unless the pilot signal is detected.
Under the circumstances, let a pilot signal generator in the base station be faulty for some reason and no pilot signal be received in each mobile station. In this event, the base station itself is put into a halt state and, as a result, a system down takes place in the CDMA system. This is because any phase reference can not be obtained on coherent detection in each mobile station due to no reception of any pilot signal.
Taking the above into consideration, a conventional CDMA system has been proposed which comprises an active unit and a backup unit each of which generates a pilot signal in a manner similar to each other. With this structure, switching is made from the active unit to the backup unit when the former unit becomes faulty and the pilot signal is generated from the backup unit to avoid interruption of the pilot signal.
However, this structure is disadvantageous in that instantaneuos interruption of the pilot signal inevitably takes place in each mobile station while the switching is made from the active unit to the backup unit. Accordingly, any other signals except the pilot signal can be neither received nor detected in each mobile station during the switching period.